F# for and while loops

last modified May 1, 2025

In this article, we show how to work with for and while loops in F#.

Loops are used to repeatedly execute a block of code. There are two kinds of loops: for and while.

F# for in loop

With for in loop, we go through a sequnce of values one by one.

main.fsx

let vals = seq { 1..5 }

for e in vals do printfn “%d” e

printfn “——————–”

let len = Seq.length (vals) - 1

for idx in 0..len do printfn “%d” (Seq.item idx vals)

In the program, we loop over a sequence of integers.

let vals = seq { 1..5 }

With seq, we define a sequence of five integers.

for e in vals do printfn “%d” e

We loop over the sequence and print each number.

let len = Seq.length (vals) - 1

for idx in 0..len do printfn “%d” (Seq.item idx vals)

In this code, we loop over a range of numbers. The values of the range are used as indexes to the sequence.

λ dotnet fsi main.fsx 1 2 3 4 5

1 2 3 4 5

F# for to/downto

With to and downto keywords, we can iterate over a range of values.

main.fsx

for e = 1 to 5 do printfn “%d” e

for e = 5 downto 1 do printfn “%d” e

The program loops over a range of 1..5 and 5..1 values.

λ dotnet fsi main.fsx 1 2 3 4 5 5 4 3 2 1

F# execute n times

A common task in programming is to execute something n times. We can do it with a for loop and a range.

main.fsx

for _ in 0..5 do printfn “%s” “falcon”

In the program, we print the word “falcon” six times. Since we do not need the values of the range, we use the _ operator.

λ dotnet fsi main.fsx falcon falcon falcon falcon falcon falcon

F# for in loop with range

Range operators can be used in a for loop.

main.fsx

for e in 1..2..10 do printfn “%d” e

for e in 10..-2..0 do printfn “%d” e

We can create ascending and descending ranges of values with different steps.

λ dotnet fsi main.fsx 1 3 5 7 9 10 8 6 4 2 0

F# nested for loops

For loops can be nested.

main.fsx

for i in [1; 2; 3; 4; 5; 6; 7; 6; 5; 4; 3; 2; 1] do for _ in 1..i do printf “*” printf “\n”

With two for loops, we create a rectangle using * characters. For each cycle of an outer loop, the inner loop is executed n times.

λ dotnet fsi main.fsx * **

---

---

---

---

---

---

---

---

---

** *

F# for loop with functions

For loops are in fact expressions.

main.fsx

open System

let rand1 () = Random(DateTime.Now.Millisecond).NextInt64(1, 10)

let rand2 () = Random(DateTime.Now.Millisecond).NextInt64(10, 20)

for e in rand1 () .. rand2 () do printfn “%d” e

printfn “——————————–”

for e = int (rand1 ()) to int (rand2 ()) do printfn “%d” e

In the example, we use the computed random values in a for range.

let rand1 () = Random(DateTime.Now.Millisecond).NextInt64(1, 10)

With random, we generate a random number between 1 and 10.

for e in rand1 () .. rand2 () do printfn “%d” e

The returned values of the two functions are used to generate a random range of values.

λ dotnet fsi main.fsx 2 3 4 5 6 7 8 9 10 11

6 7 8 9 10 11 12 13 14 15

F# while loop

A while loop is a control flow statement that allows code to be executed repeatedly based on a given boolean condition.

main.fsx

let vals = [ 1; 2; 3; 4; 5 ]

let mutable i = 0

while i < vals.Length do printfn “%d” val[i] i <- i + 1

In this while loop, we loop over the list elements in a classic imperative way.

F# funcional iteration

The for and while loops are used to create imperative loops. Functional languages often prefer to iterate over sequences with functions.

main.fsx

let vals = [ 1; 2; 3; 4; 5 ]

List.iter (printfn “%d”) vals

For instance, F# has List.iter to go over the elements of a list.

In this article we have worked with for and while loops in F#.

Author

My name is Jan Bodnar, and I am a passionate programmer with extensive programming experience. I have been writing programming articles since 2007. To date, I have authored over 1,400 articles and 8 e-books. I possess more than ten years of experience in teaching programming.